Various electronic devices require that current be supplied in a particular manner. For many electrical devices, the current must be ramped up to a peak level. Then, the current must be maintained at a second level below the peak level for a predetermined amount of time. These circuits are known as peak and hold circuits.
One example of a device that requires a peak and hold circuit is a fuel injector for an automotive vehicle. It is common, for example, that upon initially energizing a fuel injector, a peak current of approximately four amps must be reached. Thereafter, a level of one amp must be maintained during the rest of the on cycle of the fuel injector.
Commonly, a recirculating driver circuit is associated with a peak and hold circuit. A recirculating driver is a circuit that reduces the energy to be dissipated by the a module containing the peak and hold circuit. The recirculating driver circuit removes energy from the device and routes the energy back to the power supply. The recirculating driver circuit is commonly implemented using a plurality of hardware comparators. The hardware comparators are fixed in discrete components which trigger at a predetermined current, then reduce the current level to a second level below the peak level. Multiple comparators are required to hold the second level.
Several problems are associated with using hardware comparators for implementing a recirculating driver. First, implementing a peak and hold circuit is costly due to the very large number of comparators that must be used to implement such a circuit. Second, such designs may be difficult to implement due to the circuit board area required for a plurality of comparators. Third, because the comparators are "hardwired" into the system, the comparators must be configured for a particular peak and hold implementation. That is, if the characteristics needed by the electrical device are changed, for example, due to a change in the component's supplier, the comparator circuit must be changed. Such changes often require costly reconfiguration or designing and building a new circuit board.